1. Field of the Invention
The present invention relates to an imaging lens and an imaging apparatus. In particular, the present invention relates to a wide-angle imaging lens that is appropriate for use in an in-vehicle camera, a monitor camera, or the like that uses an imaging device, such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor). Further, the present invention relates to an imaging apparatus including the imaging lens.
2. Description of the Related Art
In recent years, the size of an imaging device such as a CCD and a CMOS became very small, and the resolution of the imaging device became very high. Therefore, the size and the weight of the main body of imaging equipment and an imaging lens mounted on the imaging equipment also need to be reduced. Meanwhile, imaging lenses used in the in-vehicle camera (on-vehicle camera), the monitor camera (or a surveillance camera) and the like need to have excellent weather-resistance characteristics. Further, the imaging lens needs to have wide angle of view and excellent optical performance so that an excellent view is ensured for a wide range.
Further, since it is desirable to reduce the cost for producing the imaging lens in the aforementioned fields, an optical system having a small number of lenses is needed. Conventionally, U.S. Pat. No. 7,375,906 (Patent Document 1) and U.S. Pat. No. 7,518,809 (Patent Document 2) disclose imaging lenses each composed of four lenses in the aforementioned fields.
Meanwhile, in the fields of the in-vehicle camera, the monitor camera, and the like, a demand for wider angle lenses increased in recent years. For example, lenses having full angles of view exceeding 200° became desirable. Further, as the size of the imaging device became smaller, and the resolution of the imaging device became higher in recent years, an imaging lens that has high resolution and high optical performance, and which can obtain excellent images for a wide image formation range, became desirable. However, it was difficult for the conventional optical system to satisfy such a demand, while structuring the lens system at low cost and in small size. In Patent Document 1, the full angle of view of the imaging lens is in the range of 152° to 164°, which does not satisfy the demand for a wider angle lens in recent years. Further, in all of the imaging lenses disclosed as examples in Patent Document 1, the same material is used for the second lens, the third lens, and the fourth lens. Specifically, the refractive index of the material is 1.52510, and the Abbe number of the material is 56.38. Further, since correction of lateral chromatic aberration is not intended, high resolution images are not expectable.
In the imaging lens of the Patent Document 2, both of distortion (distortion aberration) and lateral chromatic aberration are corrected in an excellent manner. However, the full angle of view of the imaging lens is in the range of 185° to 189°. Therefore, the field curvature of the imaging lens needs to be further improved to use the imaging lens for a wide angle of view exceeding 200° in combination with an imaging device, the pixel number of which has further increased.